A thermosetting resin represented by epoxy resins, unsaturated polyester resins, polyimides, and phenol resins is excellent in heat resistance, mechanical strength, or size precision and has been used widely in various fields. For example, since being excellent in mechanical strength, chemical resistance, heat resistance, adhesiveness, and electric properties, a cured product of an epoxy resin has been used widely for products required to have strength such as parts of automobiles and railroad vehicles and electric parts. Particularly, in the case such an epoxy resin cured product is used for applications such as sealing materials for optical semiconductor devices represented by light-emitting devices such as electroluminescent diodes (LED), light-receiving devices such as photodiodes, and photoelectric conversion devices such as CCD; adhesives for optical parts; and transparent coating materials, the cured product is required to be colorless and transparent and excellent in light stability in addition to the above mentioned properties.
To solve the above mentioned problems, an example disclosed is a cycloaliphatic epoxy resin which is a hydrogenated epoxy resin obtained by hydrogenating benzene nuclei of an aromatic epoxy resin with a high molecular weight, having a high hydrogenation ratio, excellent in transparency, and being solid at normal temperature (e.g., reference to Patent Document 1). However, although being excellent in transparency and light stability, this epoxy resin cured product cannot be said to be sufficient particularly as a sealing material for optical semiconductor devices and is required to have further higher resistance to cracking (toughness) in addition to high strength and high transparency.
That is, since an epoxy resin itself is insufficient in the ability of moderating heat impact stress under environments in which cooling and heating are repeated, the epoxy resin has a problem that cracks are easily caused and long term reliability is inferior if the epoxy resin is used for the above mentioned applications.
On the other hand, as one of means for improving toughness of an epoxy resin cured product, a rubber component has been conventionally compounded with an epoxy resin. Especially, employing a polymerization method in a water medium represented by emulsion polymerization, dispersion polymerization, and suspension polymerization, methods of compounding rubbery polymer particles prepared previously to be particulate are disclosed (e.g., reference to Patent Documents 2 and 3). However, so far, it has not been made possible to obtain a cured product with improved toughness while keeping transparency by compounding a rubber component with an epoxy resin.
Resin compositions for obtaining molded articles and cured products having high transparency and excellent in mechanical properties have been strongly demanded in fields relevant to building components/housing materials and electric/electronic parts and various resins have been required to have higher performance.
For example, a method of adding a diene type rubber-containing graft copolymer is employed in order to improve impact resistance of a vinyl chloride resin which is a thermoplastic resin, and a method of adding an acrylic acid ester type rubber-containing graft copolymer is employed in order to further improve weathering resistance thereof and in that case, in order to keep the excellent transparency of the vinyl chloride resin, it is common for the methods to employ a rubber component obtained by copolymerizing styrene for conforming the refractive indexes of these graft copolymers, particularly the refractive index of rubber as their main components with the refractive index (n: about 1.54) of vinyl chloride which is a matrix resin.
For these methods, it is common to employ a method of using a copolymer containing, as a main component, a monomer of which a homopolymer has a glass transition temperature (Tg) of 0° C. or lower and a refractive index (n) of 1.5 or lower, for example, butadiene (Tg=−85° C., n=1.47), butyl acrylate (Tg=−54° C., n=1.47), 2-ethylhexyl acrylate (Tg=−50° C., n=1.46), etc., and as a sub-component of which a homopolymer has a glass transition temperature (Tg) exceeding 0° C. and a refractive index (n) exceeding 1.5, for example, styrene (Tg=100° C., n=1.60).
For example, Patent Document 4 discloses a transparent thermoplastic resin composition in which core-shell type multilayer structure particles having a rubber phase with a glass transition temperature of 0° C. or lower are dispersed, characterized in that the difference of the refractive index of the rubber phase and the refractive index of a resin phase at 23° C. is within a specified range when they are measured separately and that the difference of the temperature-relevant fluctuation of the refractive index of the rubber phase and the temperature-relevant fluctuation of the refractive index of the resin phase at 23 to 70° C. is within a specified range when they are measured separately, as a method for providing a transparent thermoplastic resin composition maintaining impact resistance and mold processability which are exhibited by conventional rubber-modified transparent thermoplastic resin and having improved defects such as increase of haze and decrease of transparency due to heating. For example, the rubber layer of Example 5 in Patent Document 1 is a copolymer of 54 parts by weight of 2-ethylhexyl acrylate having the above mentioned properties and 13.6 parts by weight of benzyl methacrylate (Tg=54° C., n=1.57).
However, in the case of using a rubbery polymer which is a copolymer containing, as a sub-component, a monomer of which a homopolymer has a Tg exceeding 0° C., although it is possible to adjust the refractive index to a certain extent, the elasticity of the rubber itself is decreased and it results in failure of causing a sufficient impact improvement effect in some cases.    Patent document 1: Japanese Patent Laid-open Publication No. 2005-120357    Patent document 2: Japanese Patent Laid-open Publication No. 5-295237    Patent document 3: Japanese Patent No. 2751071    Patent document 4: Japanese Patent Laid-open Publication No. 9-048922